The present invention relates to a clutch apparatus for providing a reversible mechanical linkage between a driving motor and its mechanical load. The invention was conceived for use as part of an electric actuator in an automotive cruise control apparatus but may be useful in other applications as well.
In an automotive cruise control system which makes use of an electric actuator, such as is disclosed in U.S. Pat. No. 4,656,407, it is necessary to have a clutch mechanism which mechanically links the electric servo motor to the element which modifies the speed of the vehicle's engine. A clutch is necessary in order to limit the mechanical load against which the electric motor operates and prevent damage to the motor due to excessive current. It is further desirable in such a system for the clutch to engage or disengage upon electrical actuation so that the clutch may be incorporated into the overall control loop. A well known method for achieving this latter objective is to use electromagnetic force to engage the clutch.
In the prior art, there are found two principal designs for electromagnetic clutches. One is the friction type such as is found in the drive trains of common automobiles where torque is transmitted from one rotating plate to another by the friction existing between the two plates. In an electromagnetic friction type clutch, the plates are engaged with one another by the use of electromagnetic force rather than the use of springs as in automotive drive trains. The second type of electromagnetic clutch is the positive engagement type which uses electromagnetic force to maintain the engagement of gears, pins, or other interlocking devices. Both of these two devices can be designed to limit the torque against which the driving motor must turn by limiting the electromagnetic force which holds the clutch in engagement. When the force applied to the clutch exceeds this electromagnetic holding force, a positive engagement type clutch will disengage while a friction type clutch will slip. Both of these designs can therefore be said to be self-disengaging.
As aforesaid, it is necessary for a clutch which is part of an electrical automotive cruise control system to be self-disengaging both to prevent damage to the electric driving motor and to prevent hazardous overheating of the motor. This means, in the case of either of the two prior art devices described above, that the electromagnetic holding force must be at least as great as the maximum force which the actuation system is designed to operate against in moving the vehicle's throttle element. This maximum force is primarily determined by the throttle spring of the carburetor which the actuator must pull against in modifying the engine speed of common automobiles. Since it is not uncommon for such throttle springs to require a force in excess of twenty pounds to extend the spring, the clutch must be capable of transmitting this amount of force to the throttle. This presents a number of design problems in utilizing an electromagnetic clutch. First, since the electromagnetic holding force must be at least as great as the maximum operating force (greater in case of a friction type clutch), relatively large amounts of electrical power will be needed. The required clutch will also be physically large and will generate considerable amounts of heat. All of these factors will tend to increase the total cost of the system.
One way of overcoming the problems associated with operating against a relatively large mechanical load is to take advantage of the reduction in torque by the gear train between the driving motor and the engine throttle. Electric cruise control systems normally make use of such gear trains in order to limit the size of the required electric driving motor. By interposing the clutch between the driving motor and the first driven gear, the clutch must only operate against the same maximum torque which the driving motor is designed to operate against because of the mechanical advantage produced by the gear train. In automotive cruise control systems, however, it is not desirable to locate the clutch anywhere but between the throttle and the final driven gear of the gear train. This is because it is possible for the gear train to become jammed due to a damaged gear or the presence of foreign material. If the clutch is located upstream of the damaged gear, disengagement of the clutch will not effect a release of the throttle. For obvious safety reasons, it is desirable for disengagement of the clutch to always effect a return of the throttle to manual control.
Accordingly, it can be seen that there is a need for apparatus to overcome the problems enumerated above relating to automotive cruise control systems. It is toward these objectives that the present invention is directed.